In recent years, secondary batteries have become one of the essential and important components of personal computers, mobile phones, electric vehicles, power storages, etc., as a power source.
In particular, for use in mobile communications such as portable personal computers and PDAs (Personal Digital Assistants), further reductions in size and weight are demanded of secondary batteries. However, since LCD panel backlights and graphics-drawing control require a large amount of power consumption and the capacities of secondary batteries are inadequate under present conditions, it is difficult to make the system of secondary batteries compact or to reduce the weight of secondary batteries. As personal computers in particular have become multifunctional as a result of being equipped with a DVD (Digital Versatile Disc) drive and the like, an amount of power consumed by the personal computers is likely to grow. Accordingly, it is an urgent necessity to increase the power capacity particularly, the discharge capacity of secondary batteries when the electric cell is at a voltage of 3.3 V or more.
Further, electric vehicles that produce neither exhaust gas nor noise have become a focus of attention as global environmental awareness grows. Recently, parallel hybrid electric vehicles (HEVs) have been wining popularity. An HEV adopts a system that stores regenerative energy generated under breaking so as to use the energy in an efficient manner or uses electric energy accumulated in a battery to start the vehicle in order to increase the energy efficiency. However, since the power capacities of the current batteries are small, it is necessary to increase the number of batteries to secure a needed voltage, which results in a reduction in space inside the vehicle or deterioration of the stability of the car body.
Among secondary batteries, nonaqueous secondary batteries using a nonaqueous electrolyte such as a nonaqueous electrolytic solution have become a focus of attention because they generate a high voltage and are lightweight, so that a high energy density can be expected from them. In particular, a nonaqueous secondary battery disclosed in Patent document 1 that uses a lithium-containing transition metal oxide typified by LiCoO2 as a positive electrode active material and metal lithium as a negative electrode material has an electromotive force of 4 V or more. Therefore, it can be expected that the battery will achieve a high energy density.
However, with regard to the current LiCoO2-based secondary batteries using LiCoO2 as a positive electrode active material and a carbon material such as graphite as a negative electrode active material, their charge end voltage is normally 4.2 V or less, which is only about 60% of the theoretical capacity of LiCoO2 in this charging condition. Although it is possible to increase the power capacity by increasing the charge end voltage to be more than 4.2 V, the crystal structure of LiCoO2 disintegrates as the amount of charge increases, causing a reduction in the charge/discharge cycle life or deterioration of the crystal structure of LiCoO2. This may result in problems such as deterioration of the thermal stability.
To solve these problems, a number of attempts have been made to add dissimilar metal elements to LiCoO2 (Patent documents 2 to 5).
Further, a number of attempts have been made to use batteries in a high voltage range of 4.2 V or more (Patent documents 6 to 8). Furthermore, attempts have been made to add additives, such as a compound including two or more cyano groups, to an electrolytic solution to improve battery characteristics (Patent documents 9 to 11).